Folded horn-reflector antenna wherein primary reflector is nonreflective at portion where specular reflection to feed would otherwise occur



Nov. 8, 1966 J. c. DOLLING 3,284,802

FOLDED HORN-REFLECTOR ANTENNA WHEREIN PRIMARY REFLECTOR IS NONREFLECTIVEAT PORTION WHERE SPECULAR REFLECTION TO FEED WOULD OTHERWISE OCCUR FiledNov. 12, 1963 lNl/ENTOR J. C. DOLL/N6 QsA/wJ f ATTOR United StatesPatent FOLDED HORN-REFLECTOR ANTENNA WHERE- IN PRIMARY REFLECTOR ISNONREFLECTIVE AT PORTION WHERE SPECULAR REFLECTION TO FEED WOULDOTHERWISE OCCUR Jens C. Dolling, Morris Plains, N.J., assignor to BellTelephone Laboratories, Incorporated, New York, N.Y., a corporation ofNew York Filed Nov. 12, 1963, Ser. No. 322,826 6 Claims. (Cl. 343-782)This invention relates to microwave antennas and more particularly tofolded horn-reflector antennas which, although structurally compact,exhibit a low noise characteristic over a broad band of frequencies,

The horn-reflector antenna has received wide acceptance as a low noise,broadband microwave antenna. It is essentially a combination of aconical electromagnetic horn and a reflector formed of a section of aparaboloid of revolution, in which the feed of the antenna is located atthe apex of the horn and the apex is coincident with the focus of theparaboloidal section. Aside from use as the transmitting and receivingantenna of broadband radio relay systems and the like, these antennashave, particularly because of their low noise properties, profitablybeen used in radio astronomy and similar applications. In such uses, itis, of course, necessary that the antenna be moved in azimuth or inelevation or both in order that a moving target may be tracked. Toinsure a high order of pointing accuracy, the mechanical system utilizedto aim the antenna must be extremely responsive to electrical pointingdirection signals. If the antenna structure is of large structural mass,physical response to control signals is sluggish. In short, the smallerthe antenna, the easier it is to move accurately during a trackingoperation.

Consequently, it has been proposed to reduce substantially the physicalsize of a horn-reflector antenna by truncating the born with areflecting surface introduced along the axis of the horn andperpendicular, at the horn axis, to it. In effect, the apex of the hornis folded back upon the cone axis and is established at a point justbehind the paraboloidal reflecting surface of the antenna. A suitableaperture placed at the intersection of the cone axis and the reflectorpermits energy to travel in the general direction of the axis to andfrom an equipment room placed behind the reflector surface. With afolded configuration of this sort, the antennas length is substantiallyreduced although the reflector aperture and horn flare angle areretained at their optimum values. The configuration permits a moreeflicient structural design, particularly in terms of dynamic response.Antenna pointing accuracy is improved and the mass inertia about theazimuth axis is appreciably reduced resulting in superior response tosteering forces. With a shorter structure, the reflector suspension canbe supported more directly and can be much stiffer so that error datacompensation may be eliminated. The smaller construction also providesfor easier equipment installation, a shorter transmission line, improvedaccessibility for maintenance, and a considerable reduction in the sizeof the foundation structure. And, finally, if the antenna is to beenclosed in a weatherproof structure, such as a radome, the size of theradome and its foundation may be considerably reduced.

The numerous structural advantages of the folded hornreflector antennaare purchased, however, at the expense of electrical qualities. Inparticular, the noise temperature of the antenna is somewhat increasedover the straight cone arrangement clue, in part, to the possibility ofadditional spillover lobes, but primarily as a result of energyreflected between the truncating reflector and the apex of the conewhich, particularly during transmission, returns to the apex of the hornand the feed system waveguide.

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Although the aperture in the paraboloidal section is small as comparedwith the overall reflecting surface, the onaxis energy passing throughthe aperture from the auxiliary reflecting surface is, in a low noiseantenna, nevertheless of considerable importance.

It is the primary object of the present invention to reducesubstantially the noise temperature of a folded hornreflector antenna.

In accordance with this object, the noise temperature of a foldedhorn-reflector antenna is appreciably improved .by providing an aperturein the auxiliary reflecting sur- The invention will be fully apprehendedfrom the fol-= lowing detailed description of an illustrative embodimentthereof, taken in connection with the drawing in which:

FIG. 1 is a greatly simplified view of a truncated horn.- reflectorantenna which incorporates the features of the present invention; and

FIG. 2 is a sectional view through the elevation axis of a truncatedhorn-reflector antenna which illustrates the features of the invention.

A simplified view of a folded horn-reflector antenna which embodies thefeatures of the present invention is shown in FIG. 1 as it may bearranged for use in radio astronomy or similar applications. It includesprimary reflector 10, generally a paraboloidal surface, and a truncatedportion of a conical horn 11 coupled thereto such that the apex of thecone coincides with the focus of the reflector. A cylindrical section 25is shown encircling the antennas aperture to shield it and contribute tothe low noise characteristic of the antenna. The portion of the antennaincluding the primary paraboloidal reflector and the initial portion ofthe cone are of essentially the same configuration as that shown inPatent 2,416,675 to A. C. Beck et al. granted March 4, 1947.

The conical horn is truncated, in accordance with the present invention,by a reflecting surface 12 disposed about the longitudinal axes of thefeed horn, the cone axis, at a point along the axis which effectivelyplaces the apex of the horn and the focus of reflector 10 at a point onthe cone axis just behind an aperture 13 in reflector 10. Since thispoint on the axis lies in the plane defined by the end of the truncatedhorn, it is convenient to refere to the reflecting surface 12 as beingplaced at the plane of truncation of the horn.

Auxiliary reflector 12 may be, in the simplest case, a plane polishedsurface. To secure the utmost in size reduction, it may comprise acurved reflecting surface, e.g., a concave shaped surface, with itscenter of curvature on the cone axis and its cavity facing reflector 10.With suitable curvature, the reflective system may be made to fit Withina length equal to the beam diameter of the primary reflector, at whichlength the angular aperture, known as the flare angle, becomes aboutseventy degrees.

A conical waveguide section 14, which extends through aperture 13,terminates in a transmission line, e.g., waveguide 15, which isconnected by Way of a rotating waveguide coupling 16 to the terminalradio receiver or transmitter or both located in equipment enclosure 17.A stable rotating coupling of this sort is commonly called a chokejoint. By virtue of the choke joint, enclosure 17 remains stationary asthe horn-reflector structure rotates in elevation. In the illustration,the cone axis of the horn is horizontal so that with rotation in azimuththe horn section, paraboloidal reflector 10, and enclosure 17 move in ahorizontal plane.

The entire antenna structure is arranged to rotate in azimuth and inelevation. The primary support of the elevation structure is provided bywheel 18 located near the paraboloidal reflector end and wheel 19located near the truncated end of the conical structure. Wheels 18 and19 ride in turn on a cradle structure 20 which is arranged to rotate inazimuth. The cradle typically is of light weight space frameconstruction and supports both the antenna and equipment enclosurestructures for rotation. The total weight of the antenna structure isthus supported on bearing means, e.g., on trucks 21, only two of whichare outlined in the illustration. The bearing members ride on track 22to provide rotation about the vertical or azimuth axis.

In accordance with the invention, an aperture 30 is provided inauxiliary reflector 12 at the intersection of the axis of cone 11 withit. On-axis energy components, which after reflection duringtransmission, would again follow the axis path and pass through aperture13 in the paraboloidal reflector and enter the coupling waveguide systemas signal interference are, instead, transported by -way of awaveguiding system, coupled to reflector 12 at the aperture 30, to aradio frequency energy absorber.

The absorber may take any one of a number of forms, for

example, a labyrinth or maze, an accumulation of dissipative material, adummy antenna, or the like which terminates the waveguiding system. Theabsorber must not, however, give rise to noise since this would raisethe noise temperature of the system. A preferred dissipation systemwhich provides nearly perfect absorption, and virtually no noise orreflections, comprises an open-ended wave directive system, preferablyincluding a guide member an integral number of wave lengths long,pointing away from the pointing direction of the antenna. The directivesystem advantageously includes a reflector surface 31, e.g., aparaboloidal reflector, placed to direct energy to the sky as anabsorber, and is adjusted in length and cross-sectional dimensions toassure a reflectionless termination, e.g., by means of a tapered sectionor the like. So that the noise absorber system is always directed at aselected portion of the sky as the antenna system is moved in tracking atarget, a choke joint 33 is provided and the outer portion of theguiding system is structurally held in position.

Since the portion of the horn nearestthe apex must be smooth within atleast 30 wave lengths, and to provide shielding iagainst externalradiation, the conical wave-. guide section 14 is allowed to extend infront of paraboloidal reflector 10' to about 50m from the focus. Theextension of the horn section 14, whose aperture at the widest point istypically 30%, additionally establishes a sufficiently largeinitialradiating aperture to limit the radiated beam width and side lobes toacceptable values. The horn extension itself, of course, reducesslightly the capture area of the antenna, but the slight loss is a smallprice to pay for the drastic reduction in physical size and mass, of theantenna which is made possible by the overall folded horn construction.The more eflicient structural design, particularly in term of dynamicresponse, in turn yields improved reception due to better pointingaccuracy. Because of somewhat limited :beam width and side lo'bes,random noise components entering the waveguide 32 from the sky are noteasily coupled through the system to the receiver.

Radome 40, of any desired construction, may be used if desired toprotect the structure. By virtue of the shortened horn configuration,the antenna may be installed in a radome of considerably smallerdimensions tha'nwould be required for the nonfolded horn. Further, thenoise pattern resulting from the radome construction is held moreconstant during scanning since the antenna aperture is more nearlyconcentric with the rotational center of the radome. 1

Turning now to a somewhat more detailed description of the configurationof the antenna of the invention, FIG. 2 illustrates schematically across-sectional view of an antenna showing a primary reflecting surface10, an aperture 13 in the primary reflector, an aperture 30 in auxiliaryreflecting surface 12, and a radio frequency energy directive system 32which diverts on-axis energy away from the antenna.

In this View, it is apparent that the antenna consists of a paraboloidalreflecting section illuminated by a conical horn. The apex of the horncoincides with the focus of the paraboloidal section and the axis of thehorn is perpendicular to the axis of the primary reflector. Theparaboloidal section thus acts as a combined right angle reflector andphase eorrector for the diverging spherical wave so that waves appear atthe aperture 30 of the antenna as a plane wave front. The conical hornis truncated by auxiliary reflecting surface 12, which in effect, foldsthe truncated cone of the horn back upon the cone axis to place thevirtual apex at a point just behind the paraboloidal reflecting surface.Preferably, the horn is truncated at a point along the cone axis atwhich the diameter of the cone is approximately 225 wave lengths (k) atthe primary frequency of the antenna. This brings the apex of the coneto a point approximately 396% away from the reflecting surface at apoint just behind the paraboloidal mirror. Consequently, the initialportion of the horn which protrudes through the paraboloidal section hasa nominal dimension along the conical axis of 50A. It should be notedthat all dimensions are given in terms of wave length merely by way ofillustration since they are largely dependent on the flare angle of thehorn or angular aperture of illumination. The dimensions given areillustrative of one antenna in which the flare angle has been selectedto be optimum for both electrical and geometrical-structuralconsiderations.

Aperture 30 on the cone axis, approximately 20% in diameter, issufficiently large for removing on-axis reflections without appreciablyaltering the efliciency of the antenna or changing its primary lobepattern. It is coupled by means of choke joint 33 and preferably isfolded by means of a reflecting surface 31 to waveguiding system 32fixed with respect to the rotation of the antenna structure to a pointof high radio frequency absorption. Typically, a paraboloidal secionreflector complementary to that at the on-axis intersection of the conewith the primary paraboloidal reflector (the portion removed at aperture13) is satisfactory. It directs on-axis energy to a circular guidingsystem which, typically, is approximately 211 in diameter. It has beenfound that a length of 20 to 50A for the open ended cylindrical guide issatisfactory if 'a .paraboloilad reflector is used; a length of to i ispreferred if a plane reflector is used.

What is claimed is: v

1. An antenna which comprises, a truncated feed horn, a mirror-likereflectingsurface at the plane of truncation of said horn, a curvedprimary reflector spaced from said reflecting surface along thelongitudinal axis of said feed horn, and aperture in said primaryreflector at the intersection of the axis of said feed horn therewithwhereby the effective apex of said horn and the focus of said pri maryreflect-or coincide at a point on the side of said primary reflectorremote from said reflecting surface, an aperture in said mirror-likereflecting surface substantially on the axis of said horn, the apertureareas in said primary reflector an in said mirror-like reflectingsurface being small as compared with the areas of said correspondingreflecting surfaces, and means coupled to said aperture in saidmirror-like reflecting surface for dissipating energy incident on saidaperture.

2. An antenna as defined in claim 1 wherein said mirror-like reflectingsurface at the plane of truncation of said horn comprises a curvedreflecting surface disposed about and with its center on thelongitudinal axis of said feed horn and with its cavity facing saidprimary reflector.

3. An antenna which comprises, a *paraboloidal refiector, a truncatedhorn coupled to said para'boloidal reflector, an auxiliary reflectorsecured at the plane of truncation of said horn whereby the effectiveapex of said horn is made to coincide with the focus of saidparaboloidal reflector on the side of said paraboloida-l reflectorremote from said auxiliary reflector, an aperture in said paraboloidalreflector at the intersection of the axis of said horn therewith,waveguide feed means coaxial with said aperture in said paraboloidalreflector coupled to said remote side of said paraboloidal reflector, anaperture in said auxiliary reflector substantially on the axis of saidhorn, and means coupled to said auxiliary reflector on the side thereofremote from said paraboloidal reflector in substantially coaxialalignment with the axis -of said horn for dissipating energy directedalong the axis of said horn which passes through said aperture in saidauxiliary reflector.

4. An antenna as defined in claim 3 wherein said means for dissipatingenergy directed along the axis of said horn comprises, an open endedwave directive system coupled to said auxiliary reflector on the sidethereof remote from said paraboloidal reflector in substantial alignmentwith the axis of said horn, and means for continuously directing theopen end of said directive system toward an energy-dissipating medium.

5. A folded conical horn-reflector antenna system comprising,

a paraboloidal reflecting section illuminated by a truncated conicalhorn, the apex of which horn coincides with the focus of the saidparaboloida-l section and the axis of which horn is perpendicular to theaxis of the paraboloidal reflecting section,

an auxiliary reflecting surface supported at a point along the axis ofsaid cone at which the diameter of said cone is approximately 225wavelengths Wide at the primary frequency of said antenna, whereby thetruncated cone of said horn is effectively folded back upon said coneaxis to place a virtual apex of sai horn at a point just behind saidparaboloidal reflect ing surface,

a waveguide feed horn supported to protrude through said paraboloidalsection at the intersection thereof with said horn axis, said waveguidefeed horn having an effective aperture diameter of approximatelywavelengths, v

a symmetrically located aperture in said auxiliary re- 7 fleetingsurface with a diameter of approximately 20 wavelengths for removingon-axis energy without appreciably altering the efficiency of saidantenna or changing its primary lobe pattern, and means coupled to saidauxiliary reflecting surface coaxial with said aperture on the sidethereof removed from said para boloidal section for directing on-axisenergy to a point of high radio frequency absorption.

6. An antenna as defined in claim 1 wherein said means coupled to saidaperture in said mirror-like reflecting surface for dissipating energyincident on said aperture inv cludes a waveguide system coupled to saidmirror-like reflecting surface coaxially with said aperture, and radiofrequency energy absorbing means for terminating said waveguide system.

References Cited by the Examiner UNITED STATES PATENTS 2,682,610 6/1954King 33373 2,997,711 8/ 1961 Love 343-762 3,021,524 2/ 1962 Kompfner343781 3,133,284 5/1964 Privett 343-782 3,156,917 10/1964 Parmeggiani343--782 References Cited by the Applicant UNITED STATES PATENTS3,165,747 1/1965 Wales.

HERMAN KARL SAALBACH, Primary Examiner.

E. LIEBERMAN, C. BARAFF, Examiners.

1. AN ANTENNA WHICH COMPRISES, A TRUNCATED FEED HORN, A MIRROR-LIKEREFLECTING SURFACE AT THE PLANE OF TRUNCATION OF SAID HORN, A CURVEDPRIMARY REFLECTOR SPACED FROM SAID REFLECTING SURFACE ALONG THELONGITUDINAL AXIS OF SAID FEED HORN, AND APERTURE IN SAID PRIMARYREFLECTOR AT THE INTERSECTION OF THE AXIS OF SAID FEED HORN THEREWITHWHEREBY THE EFFECTIVE APEX OF SAID HORN AND THE FOCUS OF SAID PRIMARYREFLECTOR COINCIDE AT A POINT AND THE FOCUS OF SAID PRIMARY REFLECTORREMOTE FROM SAID REFLECTING SURFACE, AN APERTURE IN SAID MIRROR-LIKEREFLECTING SURFACE SUBSTANTIALLY ON THE AXIS OF SAID HORN, THE APERTUREAREAS IN SAID PRIMARY REFLECTOR AN IN SAID MIRROR-LIKE REFLECTINGSURFACE BEING SMALL AS COMPARED WITH THE AREAS OF SAID CORRESPONDINGREFLECTING SURFACES, AND MEANS COUPLED TO SAID APERTURE IN SAIDMIRROR-LIKE REFLECTING SURFACE FOR DISSIPATING ENERGY INCIDENT ON SAIDAPERTURE.